1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing a device.
2. Description of the Related Art
Currently, the miniaturization of circuit patterns is in progressing, along with an increase in the packing density of VLSIs. Along with this trend, the NA of a projection lens system used for a projection exposure apparatus is further increasing. To achieve this, the depth of focus that the projection lens system can tolerate in a process of transferring a circuit pattern is further decreasing. To attain satisfactory transfer of a circuit pattern, it is necessary to accurately set the entire exposure target region on the wafer to fall within the depth of focus that the projection lens system can provide.
In manufacturing, for example, a semiconductor element, a scanning type projection exposure apparatus, such as that of the step and scan scheme is used, in addition to a full-plate exposure type projection exposure apparatus, such as a stepper.
From the viewpoint of an improvement in processing efficiency, even an exposure apparatus that transfers a pattern, which is formed on an original, by exposure, using a plurality of wafer stages to improve the productivity, adopts the following scheme. That is, while a scan exposure operation is performed on one wafer stage, the surface position of the substrate is detected on the other wafer stage.
A case in which a CCD is used as a photodetector, in the scheme in which a light beam becomes obliquely incident on the wafer surface and the surface position is detected from the position of the light reflected by the wafer surface, will be exemplified below.
A charge accumulation type CCD shown in FIG. 1 accumulates a charge converted from light by a photodiode 101. A transfer gate 103 transfers the charge in the photodiode 101 to a vertical CCD 102. The vertical CCD 102 transfers the charge transferred from the photodiode 101 in the vertical direction stepwise. A horizontal CCD 104 transfers, in the horizontal direction, the charge transferred from the vertical CCD 102. An output amplifier 105 converts the charge into a voltage.
Japanese Patent Laid-Open No. 3-82282 describes a phenomenon in which a charge accumulation type CCD of this scheme cannot read out at onc time the overall amount of charge accumulated in the photodiode 101, so a certain amount of charge remains in the photodiode 101, and appears as a residual image.
FIG. 2 shows diagrams for explaining the structure of a portion that transfers a charge from the photodiode 101 to the vertical CCD 102, and the generation of a residual image. Reference numeral 111 denotes a P layer; reference numeral 112, an N layer that forms a vertical CCD 102; and reference numeral 113, an N layer that forms a photodiode. Reference numeral 114 denotes a P layer for electrically isolating pixels. Reference numeral 115 denotes a light-shielding aluminum layer for shielding the vertical CCD 102 against light.
A charge 123 is accumulated in the photodiode, as shown in portion 2a of FIG. 2. When the potential of the vertical CCD is changed from a potential 121 to a lowest potential 122, as shown in portion 2b of FIG. 2, the transfer gate is enabled. In this state, a charge 125 is read out to the vertical CCD, and serves as a readout charge. A charge 124 remaining in the photodiode serves as an untransferred charge (remaining charge) involved in the generation of a residual image. Letting Qa be the accumulated charge 123, and Qb be the untransferred charge 124, the readout charge 125 is given by Qa-Qb.
When light is accumulated in the state shown in portion 2b of FIG. 2, an accumulated charge 123 is generated, as shown in portion 3a of FIG. 3. The accumulated charge 123 is then read out to the vertical CCD as a readout charge 123, as shown in portion 3b of FIG. 3, and the charge 124 remains as an untransferred charge again. In this manner, a stable output is obtained from the CCD, as long as a predetermined accumulation and readout are repeated by performing, for example, one readout for every accumulation.
A residual image charge generated has a correlation with an accumulated charge. For example, the residual image charge increases as the accumulated charge increases, and the residual image charge decreases as the accumulated charge decreases. For this reason, when readout is repeatedly performed in a light-shielding state or a dark state, the residual image charge 124 is read out without replenishing the accumulated charge. Then, the amount of residual image charge becomes smaller than that of the residual image charge 124. FIG. 4 shows this state. Portion 4a of FIG. 4 shows a state in which light is accumulated in a light-shielding state after the state shown in portion 2b of FIG. 2. Assume that the residual image charge 124 is stored in the photodiode 101 at this time. In the state shown in portion 4a of FIG. 4, a readout charge 127 of the residual image charge 124 is then read out to the vertical CCD, as shown in portion 4b of FIG. 4, and a charge 126 remains as a residual image charge. Letting Qc be the readout charge 127, the residual image charge 126 is given by Qb-Qc. In this manner, every time the operation shown in portions 4a and 4b of FIG. 4 is repeatedly performed, the residual image charge amount decreases.
Such a phenomenon occurs in a dark state. Even when light is received by the CCD in an amount equal to that in a state other than a dark state, the accumulated charge 123 changes depending on the residual image charge amount immediately before the light reception, and the readout charge changes accordingly. In addition, the readout efficiency of the readout charge amount with respect to the accumulated charge amount changes for each pixel of the CCD. Under this influence, the output result from the CCD varies for each pixel.
To reduce such a variation in the output result from the CCD, Japanese Patent Laid-Open No. 3-82282 discloses a method of repeating operations 1 to 3:
1. The residual image charge is discharged by readout in a light-shielding state.
2. An illumination unit is turned on and then the charge is discharged.
3. All the signal charges are read out while allowing a certain amount of charge to remain in a light-receiving element by accumulating target light.
The method disclosed in Japanese Patent Laid-Open No. 3-82282 provides an illumination unit for irradiating the CCD with light, in addition to a light source for guiding a light beam to become incident on the wafer surface.
Although a case in which a CCD is used has been exemplified herein, a variation in output result inevitably occurs, irrespective of the type of photoelectric element used, as long as the efficiency of readout from it is not 100%. Also, although a phenomenon in which the untransferred charge amount changes due to readout in a dark state has been exemplified herein, it changes due to the presence of the time for which the CCD is in a dark state, because the untransferred charge discharges spontaneously. Therefore, a change in untransferred charge to be solved by the present invention includes such a change in untransferred charge due to its spontaneous discharge.
FIG. 8 shows a simple operation timing when a one-dimensional CCD sensor (to be merely referred to as a CCD hereafter) is used as a photodetector 13, serving as a position detection element. When detection light in an amount 133 is emitted by a light source 6, and received by the CCD during a CCD electronic shutter interval 131, a CCD output signal 134 is read out in the next electronic shutter interval 132. The amount 133 of detection light emitted by the light source 6 is virtually the same as the amount of detection light received by the CCD. The amount of light received in an electronic shutter interval before the electronic shutter interval 131 is read out in the electronic shutter interval 131 again, but a description thereof will not be given herein. A case in which a pixel corresponding to a certain light component that has passed through one pinhole in a mask 8 and has been received on the CCD is extracted, will be considered. In this case, the CCD output signal 134 for the CCD pixel position is as shown in FIG. 9. The position of the substrate surface is calculated from the feature values of the signal waveform shown in FIG. 9. Although the feature values of the waveform include the barycentric position and maximum position of the waveform, the barycentric position will be exemplified hereafter.
FIG. 10A, FIG. 10B and FIG. 10C show three examples of the timing at which light in the amount 133 is supplied from the light source 6 to the CCD, and the way the light is received by the CCD during the electronic shutter interval 131, and the CCD output signal is read out in the electronic shutter interval 132. FIG. 11 shows the CCD output signals for the respective CCD pixel positions, which are read out in these examples. Assume that the same position on a wafer 4 is irradiated by the light source 6, and the light reflected by the wafer 4 is received by the CCD. The CCD outputs signals even in shutter intervals other than shutter interval 132, but a description thereof will not be given herein.
A CCD output signal 135 shown in FIG. 10A is obtained when light in the amount 133 is continuously supplied to the CCD at an arbitrary interval, until immediately before the electronic shutter interval 131, and the amount of light received in the electronic shutter interval 131 is read out in the electronic shutter interval 132.
A CCD output signal 136 shown in FIG. 10B is obtained when the amount 133 of light received in the electronic shutter interval 131, after a state in which the light source 6 has been unlighted (dark state) for a time 138, is read out in the electronic shutter interval 132.
A CCD output signal 137 shown in FIG. 10C is obtained when the amount 133 of light received in the electronic shutter interval 131, after a state in which the light source 6 has been unlighted (dark state) for a time 139, is read out in the electronic shutter interval 132.
The CCD output signals for the respective CCD pixel positions in these cases are indicated by 135, 136, and 137 in 11a of FIG. 11. Letting T1 and T2 be the unlighted times (dark times) 138 and 139, respectively, T1<T2.
Portion 11b of FIG. 11 shows the differences of the CCD output signals 136 and 137 from the CCD output signal 135 for the respective CCD pixel positions, which are read out by the method shown in FIG. 10A. Letting Qd, Qe, and Qf be the CCD output signals 135, 136, and 137, respectively, differences 143 and 144 are Qd-Qe, and Qd-Qf, respectively.
The barycentric positions of the CCD output signals 135, 136, and 137 are as indicated by 140, 141, and 142, respectively. This amounts to detecting the wafer surface position as being shifted. The reason why the surface position is detected as being shifted is that the differences 143 and 144 have asymmetrical shapes and, therefore, their barycenters shift. This phenomenon is encountered because an untransferred charge is generated due to the presence of the period for which the CCD is in a dark state, and the efficiency of the charge remaining untransferred differs for each pixel. Since the untransferred charge amount due to the presence of the unlighted time, and the efficiency of the charge remaining untransferred for each pixel are constant for each CCD, the barycentric position change amount with respect to the unlighted time changes with a certain correlation, as shown in FIG. 12.
The influence of the shift in the barycentric position of the output signal and, hence, in the surface position on a semiconductor exposure apparatus will be explained next.
FIG. 6 is an enlarged view of shots 151 and 152 on the wafer shown in FIG. 5, in a semiconductor exposure apparatus. The arrow indicates the detection order on the wafer. In other words, FIG. 6 shows a state in which a shot to detect the surface position is switched from the shot 151 to shot 152.
During switching from a last surface position detection position 161 of the shot 151 to a first surface position detection position 1621 of the shot 152, the CCD is not irradiated with light, even when the light source 6 is turned on, because no reflecting surface is present. For this reason, the untransferred charge is discharged, as shown in FIG. 4. This makes only the CCD output result obtained at the surface position detection position 1621 small. Portion 17a of FIG. 17 shows the relationship between the detection values at the surface position detection position 1621 to a surface position detection position 1625 of the shot 152.
Conversely, surface position detection values, as shown in 17b of FIG. 17 are obtained when the surface positions are detected by always scanning a certain shot in the periphery of the wafer outwards (in the order of the surface position detection positions 1625 to 1621) on the wafer without detecting the surface positions, inwards in this shot, on the wafer.
As shown in FIG. 17, the surface position detection value often suffers from an error depending on the state of the CCD immediately before the surface position detection, as indicated by the surface position detection position 1621 shown in portion 17a of FIG. 17. On the other hand, it is necessary for the method shown in portion 17b of FIG. 17 to scan a shot such as that 151 outwards on the wafer, and then, return inwards on the wafer to scan the shot 152 outwards on the wafer, resulting in a decrease in productivity.
In order not to generate a dark state of the CCD, even during the switching from the surface position detection position 161 to that 1621, there has been proposed a method of projecting light from a secondary source onto the CCD, so as not to form a dark state, thereby suppressing a variation in sensitivity (Japanese Patent Laid-Open No. 3-82282). However, it is practically difficult to provide a secondary source and an optical system for forming it from the viewpoint of assuring its accommodation space and suppressing an increase in cost.